A butterfly flutters its wings in, say, England and touches off a hurricane on the other side of the world: so goes the usual explanation of the butterfly effect, a concept that as part of chaos theory expresses the way seemingly small actions can result in huge eventual changes. You could say the effect works in reverse, too: let loose a jet of CO2 in one spot on the planet, and before long butterflies in England permanently cease to flutter their wings. That’s the implication of research published yesterday in Nature Climate Change.

Of course, the idea that climate change resulting from CO2 emissions will touch off species extinctions around the globe is hardly news at this point. But, in keeping with the theme of interconnectedness, the new study is the first to calculate the interaction of climate change and a mitigation strategy – habitat restoration – on species’ chances for the future.

Sharply curtailed CO2 emissions as well as extensive habitat restoration are needed for a healthy butterfly fauna in Britain, the researchers found. Without both, many populations of drought-sensitive butterfly species are likely to disappear by 2050.

The study focused on butterflies because scientists have good records of their yearly changes in abundance, and because they are known to be sensitive to hot, dry conditions, which are predicted to increase in Britain with climate change.

The researchers combined data on the abundance of 28 butterfly species collected at 129 sites under the UK Butterfly Monitoring Scheme with historical climate data, land cover maps, and projections from 17 different climate models. They ran their analysis for four different CO2 emissions scenarios, ranging from “business as usual” conditions to emissions reductions that would limit the increase in global average temperature to 2° C at the end of the century.

The analysis identified six butterfly species – the ringlet, speckled wood, large skipper, large white, small white, and green-veined white – that are sensitive to the effects of drought and suffered major population collapses after the summer of 1995, which was the most arid Britain had ever experienced over 238 years of record keeping.

Under the business-as-usual emissions scenario, by 2100 almost every summer in the UK is predicted to be as arid as the summer of 1995. Under the 2° C warming scenario, droughts of this severity will occur roughly every 6 years.

The predicted effects on butterfly populations are similarly stark. Under business-as-usual carbon emissions, many populations of drought-sensitive species are almost certain to disappear by 2050.

With habitat restoration, these populations have between a 6 and 42 percent chance of persisting at midcentury. Similarly, curtailing emissions helps but, by itself, not by much.

For drought-sensitive butterfly populations to have a better-than-even chance of hanging on until the year 2100 requires both sharply limiting emissions and restoring habitat, the researchers found.

Their analysis also indicates that reducing fragmentation of habitat is more important for protecting butterflies than is increasing the total area of habitat – a surprising result that runs counter to recent thinking on the topic.

Although the study focused on one specific group of organisms in a relatively limited geographic area, the researchers say similar patterns would likely hold in other places and for drought-sensitive species of other taxonomic groups like birds, beetles, moths, and dragonflies. But the “beetle effect” already describes a different phenomenon that came out of England in the last century, and the “moth effect” just doesn’t have the same ring to it. – Sarah DeWeerdt | 4 August 2015